What causes "stiction" in hydraulic cylinders and how can it be mitigated?

"Stiction" in hydraulic cylinders, a portmanteau of static friction, refers to the force required to initiate movement from a stationary position, and it is caused by several factors. Primarily, it's due to the static friction between the cylinder's seals and the cylinder bore or rod surface. When the cylinder is at rest, the seals are pressed against these surfaces, creating a contact area. The force required to overcome this static friction and initiate movement is greater than the force needed to maintain movement once it has started (dynamic friction). Another contributing factor is the presence of contaminants or corrosion on the sliding surfaces. These contaminants increase the friction and adhesion between the seals and the cylinder surfaces. Additionally, prolonged periods of inactivity can exacerbate stiction, as the lubricant film between the seals and surfaces can break down, leading to increased direct contact and adhesion. High operating pressures can also contribute, as they increase the force pressing the seals against the sliding surfaces. To mitigate stiction, several strategies can be employed. Using hydraulic fluids with appropriate lubricity additives reduces friction. Selecting seals made of materials with lower coefficients of friction also helps. Honing the cylinder bore to a smooth surface finish minimizes surface irregularities that can increase friction. Regularly cycling the cylinder helps maintain a lubricant film on the sliding surfaces, preventing prolonged static contact. Implementing a dither signal, a small, high-frequency oscillation applied to the control valve, can keep the cylinder slightly moving, preventing it from remaining static for extended periods. For example, in a hydraulic excavator, stiction can cause jerky movements of the boom or arm, making precise digging difficult. Using high-quality hydraulic fluid and ensuring regular maintenance can minimize these effects."